The performance of an optical storage system is greatly related to the optical quality of the inner system and astigmatism is one of important factors for degrading the optical quality of the inner system. When the inner optical quality of the optical storage system is affected by astigmatism, the focus error signal and the tracking error signal generated by the optical storage system are easily distorted so that the servo control becomes more difficult and the unfocused condition or off-track condition may happen. Moreover, the deformed light spot of the read/write laser beam of the optical storage system resulted from astigmatism causes more errors in radio frequency signal when reading discs and poor write quality of the optical storage system.
Refer to FIG. 1, which illustrates astigmatism. An optical storage system with astigmatism is one where laser beam passes through objective lenses and propagate in two perpendicular planes have different foci. The light spot at the focal plane becomes elliptic. An included angle of 45 degrees is formed between the major axis of the elliptic light spot and tracks of the optical disc read/written by the optical storage system when the optical storage system is with astigmatism AS45. And an included angle of zero degrees is formed between the major axis of the elliptic light spot and tracks of the optical disc read/written by the optical storage system when the optical storage system is with astigmatism AS0.
Take astigmatism AS45 for example, the light spot of the optical storage system with astigmatism AS45 causes feed-through phenomenon that the focus error signal (FE) shows ripples when the elliptic light spot moves across tracks. The feed-through phenomenon becomes more obvious when the optical disc, such as DVD+R or DVD-R, has lands and grooves. When astigmatism AS45 becomes more serious, the major axis and the minor axis of the elliptic light spot differ more, so that the asymmetry of the light spot becomes more obvious. The focus error signal (FE) thus vibrates more which may lead to that the focus servo loses control. Therefore the optical pickup head with astigmatism AS45 is compensated to increase the stability of the focus servo control.
Therefore, optical pickup head manufactures compensate astigmatism AS45 and astigmatism AS0 so that read/write quality of the optical storage system are less affected by astigmatism AS45 and astigmatism AS0. FIG. 2 illustrates a conventional optical storage system. The optical storage system comprises a blu-ray (BD) laser beam source 21, a DVD laser beam source 22 and a CD laser beam source 23 for reading different kinds of discs. The BD laser beam source 21, the DVD laser beam source 22 and the CD laser beam source 23 passes through an objective lens 24 and then focuses on data layer of the optical disc 25. The reflected laser beam projects on the light detectors 26 which transfer the light signal into an electrical signal. The electrical signal is then calculated by the digital signal processor (DSP) 28 of the control unit 27, and thus a focus error signal (FE) and main push-pull signal (MPP) are acquired. Wherein the control unit 27 controls turning on/off the BD laser beam source 21, the DVD light source 22 and the CD light source 23, and an astigmatism compensated value is inputted to an astigmatism compensator 29 so that the read/write quality of the optical storage system is improved. The astigmatism compensator 29 is usually composed of liquid crystal material. The refractive index of the astigmatism compensator is varied with the inputted voltage level, thus the laser beam which passes through the astigmatism compensator 29 is compensated so that the shape of the light spot which the laser beam reaches the optical disc 25 is circular. Therefore, the optical storage system may maintain the best read/write quality.
In one conventional method, recommended compensated values for astigmatism AS45 and astigmatism AS0 are provided by the optical pickup head manufacturers. These recommended compensated values for astigmatism AS45 and astigmatism AS0 are fixed without any flexibility. However, recommended compensated values provided by the optical pickup head manufacturers are usually not optimal compensated values required by the optical storage system. Because the optimal compensated values are affected by environment variation, aging of the optical pickup heads, discs played or even usage variation (vertical or horizontal position). Moreover, the development of the optical storage system has entered blu-ray generation, the optical quality of the optical pickup head therefore demands higher standard. Thus, if only the recommended compensated values for astigmatism AS45 and astigmatism AS0 provided by the optical pickup head manufacturers are considered, the distortion of the optical signals in the optical storage system which results in the optical storage system read/write fail still happens when astigmatism AS45 or astigmatism AS0 occurs. Therefore, it is important to effectively compensate astigmatism AS45 and astigmatism AS0.